The oldest sense of antithesis refers to a language pattern that contrasts parallel ideas, as in "action, not words" or "they promised freedom and provided slavery," and antithetical originally referred to anything that was marked by such antithesis. For example, you could say "The phrase 'action, not words' is an antithetical construction."

exactly the opposite of someone or something or of each other: The idea that science is antithetical to the arts is wrong. Birth and death are antithetical.

But antithesis normally means simply "opposite". Thus, war is the antithesis of peace, wealth is the antithesis of poverty, and love is the antithesis of hate.

opposition; contrast: the antithesis of right and wrong. the direct opposite (usually followed by of or to): Her behavior was the very antithesis of cowardly. Rhetoric. the placing of a sentence or one of its parts against another to which it is opposed to form a balanced contrast of ideas, as in “Give me liberty or give me death.”